During the past three years, experimental studies have been initiated to answer the following questions: (1) what are the comparative levels of blood and tissue infectivity in sporadic and new variant Creutzfeldt-Jakob disease (CJD); (2) are the very low levels of infectivity in circulating blood associated with detectable "prion" protein (PrP) - i.e., is a blood screening test for PrP a practical goal to identify pre-clinical infection; and (3) what is the dose-response curve of infectivity in the range of temperatures between 300 and 600 degrees centrigrade, and (4) does any infectivity escape in the vented flu gas during incineration at temperatures of 600 and 1000 degrees centigrade. We have completed studies of the levels of endogenous infectivity in the blood of mice infected with an archival strain of human CJD. A critical parallel experiment is currently on test both in cynomolgus monkeys and microcebes (lemurians) with a view to observing the presence or absence of infectivity in a species more closely related to humans. The question of whether blood infectivity in "new variant" CJD is similar to that in classic disease is also under investigation both in mice and primates. The search for a test sensitive enough to detect the very low levels of "prion" protein (PrP) that might accompany the presence of equally low levels of infectivity in the blood of humans 'incubating' CJD is the subject of active research in a number of laboratories. The P.I. has just completed a review of candidate test methods (to be published later in the year), and concludes that several methods have promise. However, one such method - competitive antibody inhibition followed by capillary electrophoresis - although apparently successful in detecting PrP in scrapie-infected sheep, has not in our hands proven to be capable of detecting PrP in the blood of humans with sporadic or variant CJD, or in the blood of chimpanzees infected with human strains of CJD. The other principle aspect of infectivity that we are studying is the heat resistance of CJD-like agents. Many years ago, we found that a hamster-adapted strain of scrapie was not entirely destoyed by heating at a temperature of 360 degrees centigrade. This surprising result was generally explained away by the fact that the sample had been freeze-dried under vacuum and was therefore protected both by dehydration and the absence of oxygen. In a repeat experiment using the same scrapie strain in a macerate of fresh, untreated brain, in an open crucible, and have again found that a substantial residue of infectivity persists after heating 15 min at 300 degrees centigrade, and a miniscule amount of infectivity after exposure to 600 degrees centigrade. We have recently completed two sets of heat inactivation experiments: 1) muffle oven heating of infected brain tissue at 50 degree intervals between 300 and 600 degrees centrigrade, and 2) experiments mimicking incineration conditions at 600 and 1000 degrees, in which we have collected both the ashed samples and trapped exhaust gases, both of which will now need to be assayed for infectivity by inoculatio